Colloidal co-assembly

Nanoporous architectures use periodic arrays of hollow spaces to generate the intense structural colors of butterflies, beetles, and birds, enhance the mechanical stability of bones, and facilitate gas exchange through egg shells.

Analogous synthetic periodic nanoporous structures, known as inverse opals, offer a compelling materials strategy for use in optics as well as in fields ranging from catalysis and energy storage to tissue engineering. While inverse opals and other 3D photonic structures can be produced by top-down processes, a much simpler, lower cost approach to generating uniform pore size and order is to use self-assembling colloidal spheres to construct a patterned, periodic colloidal crystal, or opal, which then acts as a sacrificial template for self-assembling the porous structure. However, this technique has been plagued by uncontrolled crack and defect formation over the length scales required for most applications. We have discovered that taking a simpler approach - letting colloids and a silicate sol-gel precursor co-assemble in one step rather than sequentially – generates highly ordered, crack-free, multilayered inverse opal films on the scale of centimeters. 

Publications

Phillips KR, Vogel N, Hu Y, Kolle M, Perry CC, Aizenberg J. Tunable Anisotropy in Inverse Opals and Emerging Optical Properties. Chem. Mater. [Internet]. 2014;26 (4) :1622-1628. Publisher's VersionAbstract

Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the possible structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method for introducing anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol−gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure. Further, we prevent the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy and infrared spectroscopy studies clearly identify increasing degrees of sol−gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature.

Media Coverage

The dynamics of evaporative patterning, Harvard press release, October 6, 2015. 

Controlling Evaporative Patterning Transitions, American Institute of Physics, September 29, 2015.